What is an Ideal Diode?

Ideal diodes are semiconductor switches that allow current to pass in one direction but block current trying to flow in the opposite direction. They offer infinite resistance to reverse current flow and zero voltage drop.

Compared to conventional diodes, ideal diodes can improve efficiency by reducing power dissipation. This is particularly true in bridge rectifiers.

Unlimited Forward Current

An ideal diode is a semiconductor switch that acts like a perfect conductor when voltage is applied forward biased and an insulator when voltage is applied reverse biased. As a result, when current is flowing through a real diode with no reverse voltage, it behaves as if it is a short circuit. But when a reverse voltage is applied, it will offer infinite resistance and inhibit the current flow.

While there are many ratings for a diode, the most important is the current-voltage (i-v) curve. This characteristic is what defines the operation of a diode and how it performs in a circuit. Unlike conventional diodes, the i-v curve of an ideal diode is non-linear. This is because a real diode will always have some internal resistance that cannot be zero.

This resistance, called bulk resistance, is a function of the size and temperature of the diode package, and the thermal transfer capability of its materials. It limits the maximum power dissipation, or PD, of the diode. The PD is calculated as the product of diode current and diode voltage drop, or I2R, and divided by the junction surface area. This figure is also limited by the operating junction temperature (TJ), which must be kept low to maintain good diode performance and long life. This is why the ideal diode is so desirable in electronic design.

Unlimited Resistance to Reverse Voltage

An ideal diode offers infinite resistance when reverse-biased. This can be directly attributed to its previous property that it mimics the behavior of a perfect insulator. Since current can never flow through a perfect insulator, an ideal diode will offer infinite resistance to the flow of charge carriers regardless of the magnitude of the reverse voltage applied.

A real diode, on the other hand, does conduct a small amount of reverse current known as leakage current. This is due to the existence of barrier potential V0 (0.7 V for silicon and 0.3 V for DC converter germanium) which is needed to stop holes and electrons from crossing the junction.

Therefore, in order to avoid any leakage current from the p-side to the n-side of the diode, the voltage on the n-side must be much higher than that on the p-side. This can be achieved using either comparators or linear servo amplifiers, which control the voltage drop across the n-channel MOSFET source-to-drain with a precise value.

Ideal diodes are crucial components in electronic circuits because they can improve the efficiency and reduce power dissipation by acting as a short circuit in the forward direction and an open circuit in the reverse direction. They also eliminate the issues of heat loss, hysteresis, and capacitance. They are a key component of rectification circuits, voltage regulators, and other high-speed power electronics.

Unlimited Breakdown Voltage

An ideal diode has no reverse voltage drop (also known as breakdown voltage). Conventional diodes are not perfect insulators and will conduct some amount of reverse current. This reverse current is known as leakage current. When the reverse voltage on a conventional diode exceeds the breakdown voltage, the junctions will break down and begin to conduct a large amount of current. The ideal diode has no breakdown voltage, which means it will never conduct any reverse current.

The actual reverse voltage of a diode depends on the doping and material properties of the semiconductor used to create the component. This is one of the many electrical characteristics that makes it difficult to replicate in an ideal diode model.

SPICE-based circuit simulators like LTspice include a standard model for a diode that can be used in basic simulations. The model file includes predefined electrical parameters for the diode, which can be copied into a new component model for different diodes using datasheets or measurements.

This standard model is an approximation that works reasonably well in most circuits. However, for more accurate calculations it’s important to use a more realistic model that accounts for the reverse saturation current and other details of the diode’s behavior. This is especially true for applications that involve transient analysis or parameter sweeps. To achieve more accurate results, it’s recommended to use a multi-parameter model for the diode that takes into account its forward and reverse breakdown voltage as well as its forward current limit and reverse leakage current.

Circuit Symbol

An ideal diode behaves like a perfect semiconductor switch that conducts when potential is applied forward biased and acts as an ideal insulator when voltage is applied reverse biased. The circuit symbol of an ideal diode is a triangle shape against a line with the direction the current flows marked by an arrow. The terminal entering the flat edge of the triangle represents the anode and the one leaving it is the cathode. Current will flow in the direction the arrow is pointing but not the other way.

This ideal behavior of a diode is because of its first property which states that when the anode is exposed to a forward voltage it conducts immediately. This is in contrast FPGA Modules to conventional diodes where a certain threshold voltage must be reached before they start conducting.

In addition, the ideal diode has no barrier potential so it never performs a reverse current called leakage current. This is in contrast to conventional diodes which do allow a small amount of current to flow for a short time known as the reverse recovery current. This reverse current is dependent on the temperature of the semiconductor and the operating voltage so it will vary slightly from manufacturer to manufacturer and application to application. However, the general rule is that the reverse recovery current will be very small compared to the forward current rating of the diode.